Formation of contact and multiple cyclic cassiterite twins in SnO2-based ceramics co-doped with cobalt and niobium oxides.

Contact and multiple cyclic twins of cassiterite commonly form in SnO2-based ceramics when SnO2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO2 on CoNb2O6 and Co4Nb2O9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO2 grain growth; CoNb2O6 forms at ∼700°C and Co4Nb2O9 at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO2 domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb2O6 grains can only result in the formation of contact twins, the Co4Nb2O9 grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO2 grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO2 grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb5+ and Co2+ ions into the SnO2 matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.

Twinning in SnO2-based ceramics doped with CoO- and Nb2O5: morphology of multiple twins revealed by electron backscatter diffraction.

Electron backscatter diffraction (EBSD) was used for the analysis of multiple cyclic twins in cassiterite (SnO2), which form during sintering of SnO2 with small additions of CoO and Nb2O5. Grain misorientation analysis has shown that about one third of all grains contain {101} twin boundaries (TBs). The majority of these grains are contact twins, whereas a small fraction of grains are multiple, mainly cyclic twins. A procedure was developed in MTEX [Bachmann, Hielscher & Schaeben (2010). Solid State Phenom. 160, 63-88] for automated identification of crystallographically different types of cyclic twins and found two main types: coplanar twins composed of three or four domains with a common [010] axis and alternating twins composed of three to seven domains oriented along the [111] axis. Both types of cyclic twins have a characteristic common origin (nucleus) of all TBs, which is positioned eccentric relative to the grain section and the cycle is closed with a shorter non-crystallographic contact between the first and the last twin domain. The morphology of cyclic twins suggests that they form by nucleation in the initial stages of grain growth. The average size of twinned grains increases with the number of twin domains indicating the influence of TBs formation on the growth of composite grains.